Seismic Waves Show Which Sport's Fans Rock Hardest

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NASCAR has loud fans and even louder engines, but can it beat the
"Beast Quake?"

Football, NASCAR and their rowdy, roaring crowds faced off in a
head-to-head battle this year to see which sport hits highest on
the seismic charts, scientists reported Dec. 18 at the American
Geophysical Union's annual meeting in San Francisco.

Not to be outdone, this year the Texas Motor Speedway asked
seismic experts from Southern Methodist University in Dallas to
record the Duck Commander 500 race. It's a typical NASCAR race,
with 43 stock cars roaring around a 1.5-mile (2.4 kilometers)
track and twice the number of fans as a Seahawks game.

(The Seahawks play at CenturyLink Field, formerly the Seahawks
Stadium, with a 67,000 seating capacity. The Texas Motor Speedway
seats more than 138,000 people.)

"The owner wanted to be able to say his race had larger ground
motions than the Seattle Seahawks," joked Brian Stump, a
seismologist at Southern Methodist University and co-leader of
the project. More seriously, Stump and other scientists are
interested in monitoring large crowds with seismic and acoustic
signals transmitted through the earth and the air. And large
structures such as stadiums, bridges and tunnels
have natural frequencies that can change when something is
amiss, such as unwanted cracks. As such, monitoring the
vibrations is a way to detect unseen damage to these imposing
structures.

"We're thinking about how we can use these techniques to monitor
a number of sources," Stump said.

On April 7, 2014 — a day late due to rain — a network of
listening devices installed on volunteer time by Stump and his
students and colleagues switched on both inside and outside the
racetrack stadium. The instruments were set up to record
everything from infrasound, or sound below human hearing, to
explosion-level
noise, and from earthquake-strength shaking to the weakest
tremors.

Winner takes it all

So who rocks hardest?

It turns out it's not a fair contest.

"[NASCAR] is apples to oranges from a Seahawks game," Stump told
Live Science. "It's a completely different phenomenon."

But a NASCAR race is an endurance event, without much beyond
crashes to bring the crowd together on their feet. No seismic
signals came from the crowd during the race, Stump reported.

Instead, the strongest vibrations were between 20 hertz to 100
hertz, five times higher in frequency than the signals seen at a
Seahawks game, Stump said. These tremors came from the racecar
engines, he said. Their deep bass rumbling made the ground
vibrate, called acoustic-to-seismic coupling.

It's possible that the Seahawks crowd also makes higher-frequency
noise in the same range, but the seismic instruments, or
seismometers, used in Seattle aren't designed to detect those
frequencies, said John Vidale, a seismologist with the Pacific
Northwest Seismic Network. Vibrations from the stadium travel
through the earth at about 2.5 hertz and 5 hertz.

"Our instruments are looking for earthquakes,"
Vidale told Live Science. "And we have lower frequencies because
we're seeing a whole football stadium resonate, whereas these
cars are screeching at 20 hertz and higher," Vidale said.

The higher frequencies detected at the Texas race can't travel
far through the earth, so little in the way of race noise was
picked outside the speedway. The strongest signal that appeared
on the researchers' distant instruments was from a magnitude-4.5
earthquake in Oklahoma that rattled the stadium during the study,
though it was not strong enough to be felt by race fans in
Texas.

Inside the Speedway, the Texas researchers and their instruments
could also "see" weekend fireworks, two helicopters tracking the
race, machines drying the track after a Sunday rain, a fiery
crash, and green flag and yellow flag laps. Stump said the
overall experiment was a great success.

"We can directly relate the recordings to things going on in the
race," Stump said. "Seeing the physics of the race itself and
pairing it with the waveforms was a great educational
experience."